ACTIVITY pubs.acs.org/jchemeduc
JCE Classroom Activity #109: My Acid Can Beat Up Your Acid! Alice Putti* Science Department, Jenison High School, Jenison, Michigan 49428, United States
bS Supporting Information ABSTRACT: In this guided-inquiry activity, students investigate the ionization of strong and weak acids. Bead models are used to study acid ionization on a particulate level. Students analyze seven strong and weak acid models and make generalizations about the relationship between acid strength and dissociation. KEYWORDS: High School/Introductory Chemistry, Curriculum, Interdisciplinary/Multidisciplinary, Collaborative/Cooperative Learning, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Acids/Bases FEATURE: JCE Classroom Activity
M
ost high school chemistry activities focusing on acids typically involve the reaction of an acid with a base. Usually, students are required to make observations, such as the color change of an indicator or formation of a gas, to determine the properties of an acid.1 3 While these activities are fun for students, they only focus on the macroscopic view of acids. As a result, students often have a poor understanding of acid behavior. The purpose of this guided-inquiry activity is to study the difference between strong and weak acids on a particulate level. This activity uses plastic beads as models for molecular and ionized acids. The models represent seven different strong and weak acids (Table 1). By noting the differences in the models and counting the amount of beads representing molecular acids versus ionized acids, students learn about acid ionization and develop an understanding of the difference between a strong acid and a weak acid.
writing reactions from the models, students should also learn how to write an ionization equation.
’ INTEGRATING THE ACTIVITY INTO YOUR CURRICULUM This activity can be used to introduce the difference between the ionization of a strong acid and weak acid. It also focuses on the ionization of a diprotic acid versus a monoprotic one. It could be used in any introductory high school or college course and can be completed in one, 60-min class period. Before using the activity, students need to be familiar with acid naming and formula writing. They should also be familiar with reversible reactions. Although this activity focuses solely on acid ionization, the concept can be applied to explain the difference between strong and weak bases. ’ ABOUT THE ACTIVITY Students compare the ionization percentage of strong and weak acids by exploring particulate models made from beads in Petri dishes. Each Petri dish contains models of 10 acid molecules or acid ionization products to represent one of each of the seven acids (Table 1). In the monoprotic acid model, single beads represent acid ionization products and glued beads represent bonded acid molecules (Figure 1). In the diprotic acid model, two glued beads and a single bead represent acid ionization products; an acid molecule is three beads glued together (Figure 2). Students count the number of ionized particles out of 10 possible acid molecules in order to determine the
’ BACKGROUND Many students do not understand the molecular difference between a strong acid and a weak one. Most know that a 0.1 M strong acid has a pH around 0 or 1 and a weak acid of the same concentration has a pH around 5; however, students are usually unable to explain why the pH values are different.4 Even fewer students understand the difference in behavior between a monoprotic and a diprotic acid. Two common misconceptions regarding strong or weak acids are that sulfuric acid is a strong acid because each molecule has two hydrogen ions, and that all diprotic acids completely ionize in water. In this activity, students will see that some diprotic acids are weak acids (H2CO3) and therefore do not completely ionize in water.5 Moreover, by Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
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Table 1. Acid Model Assembly Details for Seven Specific Strong and Weak Acids Used in This Activitya Petri Dish
a
Acid
Type of Acid
Bead Colors
Diprotic Acid: 3 Bonded (2B + 1 other)
Monoprotic Acids: 2 Bonded (1B + 1 other)
Individual Beads
1
HNO2
Weak
Blue/Pink
0
8 (B P)
2B, 2P
2
HBr
Strong
Blue/Red
0
1 (B R)
9B, 9R
3
H2SO4
Strong
Blue/Orange
1 (B Or B)
9 (B Or)
9B
4
HF
Weak
Blue/Pink
0
9 (B P)
1B, 1P
5
H2CO3
Weak
Blue/Orange
9 (B Or B)
1 (B Or)
1B
6
HClO4
Strong
Blue/Green
0
0
10B, 10G
7
H2SO3
Weak
Blue/Yellow
8 (B Y B)
2 (B Y)
2B
H+ = blue bead. Colors used for these 7 models: B = blue; P = pink; R = red; Or = orange; G = green; Y = yellow.
Figure 2. Example of a weak diprotic acid model, with acid partially ionized (20% of the molecular acids ionized).
seven groups. Give each group one Petri dish and have them fill out the table for that acid in the student guide (see the online Supporting Information) and rotate the groups so that every group examines each model. When completing the table, make sure students pay attention to whether the acid has two or three beads. While monitoring the activity, make sure that students correctly write the formulas and ionization equations (including correct charges on ions). A common mistake is incorrectly counting the number of intact acid molecules and ionized acids. For example, students may not recognize that a blue bead (H+) and a red bead (NO2 ) represent one ionized acid molecule and incorrectly count it as two separate acids (Figure 1). This is an opportunity to review acid definitions with students. Students may have particular difficulty with the diprotic acid ionization reactions. Even though the diprotic acid model (Figure 2) indicates that the ions should be H+ and HSO3 , students often incorrectly write the formula as H+ and SO32 . This error might occur because when students write the formula of sulfurous acid by balancing the charges of sulfite and hydrogen, they often do not consider the HSO3 1 ion as a possible product of the ionization. If students are unfamiliar with the hydrogen sulfate ion, it may be necessary to explain why the charge is 1 and not 2 . Instructors should also make sure that students correctly balance the charges in all the reactions so that the number of positive charges (hydrogen ions) matches the negative charges (anions). After students have studied the models, give them a list of strong and weak acids (see Table 1) in order to complete the results and analysis section in the student activity worksheet (see the online Supporting Information). Using the information provided, students should be able to complete the rest of the questions in this section. Strong acids are known to ionize completely (95 99%) and weak acids only partially (less than 5%). Because each model has only 10 molecules or ions, the strong acids ionize 90 100% and weak acids 10 20%. Though these percentages are incorrect, students can still understand how labeling the acid strength depends on the degree of ionization.
percentage of ionized acid particles. After examining all seven Petri dish models, students then compare their data percentages in a list of strong and weak acids. The student activity worksheet, available in the online Supporting Information, contains an “example acid” with a drawing and several questions that students work as a class to answer before starting the activity. At this time instructors can discuss how to count the whole acid beads and the ionized acid beads, as well as writing the balanced ionization reactions. If aiming to complete the entire activity in one period, divide students into
’ HOW TO MAKE THE MODELS A complete classroom set of models includes one Petri dish for each of the seven acids listed in Table 1. The seven Petri dishes are shared among the entire class. To make the complete set of acid models, you will need 170 pony beads (69 mm): 100 beads of a single color to represent hydrogen (blue is used in this guide); 70 other colored pony beads (10 of each of 7 different colors); 7 Petri dishes, and glue. The beads can be purchased at hobby stores and plastic Petri dishes can be ordered from most scientific catalogs. Bead glue or a hot glue gun can be used to
Figure 1. Example of a strong monoprotic acid model, with acid completely ionized (90% of the molecular acids ionized).
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“bond” the molecules. The contents of one Petri dish represent the products upon ionization of one acid species. To avoid student confusion, make all the models of H+ ions the same color throughout the activity. The easiest method of assembly is to place one bead on the sharpened end of a pencil. Apply the adhesive to the bead and then attach the second bead. Hold about 20 s. Apply a small amount of adhesive to the outside where the two beads are attached. Pop the beads off the pencil onto a piece of cardboard. Rotate the beads periodically to prevent them from sticking to the cardboard while the glue is cooling and hardening. Assemble the models in the Petri dish according to the Table 1. Label each Petri dish with a number. Use tape to seal the Petri dishes shut. It takes approximately 1 h to make one complete set of acid models. Once made, the models can be reused for years.
’ GOING FURTHER Two additional follow-up activities are available online.6 The first activity is a study of the difference between concentrated and diluted acid. This follow-up activity also uses the beads (individual and glued) along with beads meant to represent water molecules (clear, colorless acrylic beads). Using the criteria established in this part of the lab, students determine the strength of an acid. Then, water beads are added to the acid in order to see how the concentration and strength are affected by the addition of water. The second activity is a traditional investigation of weak and strong acids when mixed with universal indicator. At the end of both follow-up activities, students answer questions that connect back to the original model activity. ’ ASSOCIATED CONTENT
bS
Supporting Information Student activity worksheet; answers to student questions. This material is available via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Special thanks to Deborah Herrington, Ellen Yezierski, and the Target Inquiry Program at Grand Valley State University. ’ REFERENCES
^ . F. S. S.; Piedade, F. M. J. Chem. (1) Carvalo, A. P.; Mendonc) a, A Educ. 2002, 79, 1464A–1464B. (2) Holt Chemistry: Lab Generator CD ROM; Holt Rinehart Winston: New York, 1996. (3) Wilbraham, A. C.; Staley, D. D.; Matta, M. S. Laboratory Manual Teacher’s Edition: Prentice Hall Chemistry; Pearson Prentice Hall: Boston, MA, 2004; 247 254. (4) Furio-Mas, C.; Calatayud, M.-L.; Barcenas, S. L. J. Chem. Educ. 2007, 84, 1717–1724. (5) Ogata, P.; Schreck, J.; Willis, C. Acids and Bases: A Sourcebook Module. http://www.okstate.edu/jgelder/acidandbase.html (accessed Jun 2011). (6) Putti, A. Target Inquiry Web Site. http://www.gvsu.edu/ targetinquiry/ (accessed Jun 2011). 1280
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